Introduction & Objective: Undoubtedly reproduction as a means of human survival and recurrent spontaneous abortion being the most prevalent complication of pregnancy with a three percent outbreak and an influential problem on human life have always been the subject of interest among researchers. Different factors are involved in recurrent spontaneous abortion among which immunological disorders are considered the most important ones . Immune cells in uterine endometrium specially natural killer (NK) cells at the menstrual cycle and during pregnancy change considerably indicating their important role in reproductive cycle. The aim of the present study was to investigate and compare NK cells in the peripheral blood of nonpregnant women with recurrent abortion, and non pregnant women with normal pregnancy on the basis of flow cytometry development technology and availability of monoclonal antibodies.

Materials & Methods: In a case-control study 30 samples of peripheral blood from nonpregnant women that had three or more consecutive spontaneous abortion with no other medical problems and anatomical disorders (patient group) were obtained. 30 samples of peripheral blood from normal nonpregnant women (control group) were also obtained. All blood samples, after lysis of red blood cells, were stained with two type of monoclonal antibodies anti CD16 and anti CD56 (NK cell specific markers) and then analyzed by flow cytometry. The data from the two groups were compared using t test.

Results: NK cells significantly increased in women with recurrent abortion compared with normal nonpregnant women indicating significant differences in the mean average of CD56+CD16+ population between the patient group and control group (P =0.026). The mean average of two CD56+ and CD16+ populations significantly increased in the patient group in comparison with the control group (P<0.05).

Conclusion: The increase of NK cells in peripheral blood followed by cytotoxicity function of the cells, particularly in relation to emberyo trophobast cells at the pregnancy cycle, can contribute to the recurrent abortion etiology. More investigations including studying the function of these cells, secretory cytokines and their receptors in recurrent abortion, and other disorders of reproductive system such as infertility, endometriosis, preeclampcia are suggested.